1. Field of the Invention
The present invention relates to a filter type trapping agent for volatile cesium compound and a trapping method for volatile cesium compounds thereof.
2. Description of the Related Art
Cesium, the high level radiation gaseous waste, is generated from a spent fuel management process (such as an oxidation process, an electrolysis process, a fluoridation process, and a chlorination process of a nuclear fuel irradiated in nuclear reactors or spent fuel), a spent fuel solidification process, and a high level radiation liquid waste roasting process. It is also generated from a process of roasting the high level radiation liquid waste that results from a reprocessing process, and a solidification process. In many processes dealing with the spent fuel, not only cesium (Cs) but also diverse semi-volatile fission gases like I, Tc, Ru, Te are generated depending on heat processing temperature and air atmosphere. Among these gases, cesium with high level radioactivity and heat radiation should not be released to the environment. Therefore, technology development for selectively trapping cesium with safety is urgently required.
In the conventional art, a condensation method of producing gaseous cesium into fine aerosol particles and removing them using a high efficiency filter was used to effectively remove gaseous cesium. However, the method had a great danger for radiation and the disposal problem was an issue as the method did not involve a technology of trapping the radioactive cesium having a great chemical reaction into a chemically stable form and a substance with high leach resistance. Also, a method has recently been developed for generating fly ash produced by coal-fired steam plants into a filter, making the fly ahs react with gaseous cesium, manufacturing cesium alumino silicate, and trapping cesium. However, quality control could not be made as the composition of fly ash varied depending on change in the kind of coals used in the coal-fired steam plants. That is, producing pollucite (CsAlSi2O6) is hard because irregular chemical composition occurs when the fly ash is generated into a filter. Furthermore, selectively removing cesium is hard to be achieved because of diverse chemical compositions such as, CaO, MgO, TiO2, SO3, Na2O, K2O, BaO, PbO, MnO20, included in the fly ash.
Actually, not only cesium but also technetium-99 is simultaneously trapped in the fly ash filter. If technetium-99 as well as cesium is trapped in the fly ash filter, cesium, which is short-lived radionuclide with high level of radioactivity and heat dissipation (137 Cs: 30.2 years) and the technetium which is long-lived radionuclide (99 Tc: 210,000 years) are simultaneously disposed when the waste filter in which the cesium and the technetium are trapped together is permanently disposed. Therefore, effective disposal and recycle of cesium and technetium cannot be anticipated.
Metakaolinite (Al2O3.2SiO2), bentonite (Al2O3.4SiO2.6H2O), and pyrophillite (Al2O3.4SiO2.H2O) that exist in nature are also used as cesium trapping source materials. However, clay such as the metakaolinite with mole ratio of Si/Al=1.04, the pyrophillite with 2.07, and the bentonite with 3.53 have chemical compositions unsuitable for generating pollusite (CsAlSi2O6), a stable form of cesium. In addition, when these materials are generated into a filter, the intensity becomes so weak that the shape is hard to be maintained because have high hygroscopicity. Korean Patent Registration No. 10-0184254 relates to a gaseous cesium (Cs) trapping agent using waste catalyst for fluid catalytic cracking produced during a heavy oil upgrading process in petrochemical plants. However, as the fluid catalytic cracking catalyst contains zeolite absorbing moisture, complete plasticity can not be achieved when generated into a filter. Therefore, it is hard to process and shape the trapping agent into a conveniently-operable filter. In addition, the gaseous cesium is trapped as a powder form after the waste catalyst for fluid catalytic cracking is performed. Therefore, a blockage in the filter wall and pressure drop due to non-oxidation, and low contact efficiency of gas and solid channeling can lead to an operation problem during off-gas treatment. Currently, in order to remove the gaseous cesium by using a fly ash and a waste FCC catalyst, the operation temperature must be maintained at 600-1300 and 500-1000. If the removing work is done in an off-gas treatment system in nuclear power facilities, the off-gas cannot react to low temperature, thus requiring additional heating equipment.
For the sake of completion, when the inventors were conducting research on the method of trapping cesium compound, we developed a method of trapping volatile cesium compound by using a filter type trapping agent including 40-65% by weight of silica, 15-30% by weight of alumina, 5-15% by weight of iron oxide, 1-15% by weight of molybdenum oxide, 1-10% by weight of chromium oxide, and 1-10% by weight of vanadium oxide.